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Magnetic and Electrical Separation, Vol.7, pp.145-161 Reprints available directly from the publisher Photocopying permitted by license only (C) 1996 OPA (Overseas Publishers Association) Amsterdam B.V. Published in The Netherlands under license by Gordon and Breach Science Publishers SA Printed in Malaysia UPGRADING OF RAW PERLITE BY A DRY MAGNETIC TECHNIQUE D. HERSKOVITCH AND I.J. LIN Mineral Engineering Research Haifa 32000, Israel Centre, Technion, (Received November 8, 1995, in final form December 20, 1995) Abstract Perlite of composition SiO2 73.6%, A1203 12.4%, Fe203 1.25%, MgO 0.15%, Na20 and found to contain 89.4% of 2.99%, K20 4.18%, HO + 3.17%, H20-0.2%, amorphous phase and 10.6% of crystalline phase, the latter consisting of feldspars, biotite, quartz, magnetite and chlorite, was studied. Due to its largely amorphous nature, chemical composition and melting behaviour, perlite can be used in the manufacture of ceramics. For the glass industry, on the other hand, an impediment, albeit the only one, is the iron content, so that the iron-bearing minerals must be removed, which can be accomplished by dry magnetic separation. Laboratory and pilot-plant separation experiments carried out with these minerals showed that the FeO3 content cannot be reduced to below 0.65% owing to small inclusions of iron-bearing minerals that remain in the other minerals, or in the amorphous particles. Perlite with an FeO3 content below 0.7% and grain size of less than 1 mm can, however, be used in the production of eoloured glass containers. INTRODUCTION Perlite is a hydrated volcanic glass of rhyolitic composition containing 1 to 5 % of combined water. It belongs to the same family of volcanic glasses as obsidian, pitchstone, pumice and scoria. Its most important property is its ability to expand to about 20 times its original volume when heated to a temperature within its softening range. The result is Wtexpanded perlite", a product of low density, low thermal conductivity and high sound absorption. 145 146 D. HERSKOVITCH AND I.J. LIN Thanks to its characteristics expanded perlite is used in a variety of industrial and construction applications, as well as in agriculture. Concomitant with the growing interest in the expanded material, the possibility of using crude perlite as a raw material in some applications is also beginning to draw attention of researchers. Raw perlite, of different fractional grain sizes and bulk densities, has already been successfully tested in the ceramics industry as a component of ceramic bodies for floor and wall tiles [1], as an inorganic additive to serve as a drying agent, or as a sintering auxiliary in a ceramic body [2], as a mineral additive in the technology of self-glazing and pigmentation of ceramic tiles [3]. A very important application of raw perlite is its use as a raw material in the synthesis of zeolites [4]. Due to its physical characteristics- uniformity coefficient, hydraulic size, density and porosity, perlite constitutes a good filter material [5]. Due to SiO2/A120 ratio, alkaline content, homogeneity and amorphous structure of perlite, two very new applications are as a mineral additive instead of clay in wet phosphoric acid process [6] and as an additive in the production of ceramic glass [7]. PROPERTIES OF PERLITE The most important physical properties are.listed in Table 1. The perlite studied originated in the island of Milos and was supplied by Habonim Industries, Moshav Habonim. It has a light gray colour and a vitreous luster. The presence of vesticules is the most obvious feature. Phenocrysts of quartz and feldspar measuring 1 to 2.5 mm are imbedded in the glass matrix. Biotite flakes can also be observed with the naked eye. The perlitic glass exhibits a perfect and encircling fluidal phenomena (Fig. 1). The granulometric distribution of the crushed perlite is illustrated in Fig. 2. Most of the particles are contained in the 20 to 70 mesh size range. A microscopic analysis of various sieved fractions indicated that the feldspars and quartz are present in all granulometric fractions. The chlorite is mainly contained in the large D. HERSKOVITCH AND I.J. LIN 14s 35 3O 25 0 ]0 5 0 0 104 oo 140 210 300 70 50 500 840 /m 35 20 Mesh Particle size Granulometric distribution of perlite Fig. 2 (+ mesh), while biotite and magnetite are concentrated in the smaller fractions (- 70 mesh). They appear as individual crystals or as inclusions size fractions 35 in the other minerals and in the glass grains. Biotite is also abundant in the -50+ 70 mesh fraction. Chemically, crude perlite is a hydrated aluminium silicate. A series of chemical analyses of perlite ores of different origins is given in Table 2. It will be seen that perlite is characterised by a high silica (_> 65%) and alumina (11 to 17%) content. Noteworthy is also the alkaline content (7 to 8.5%). UPGRADING OF PERLITE BY A MAGNETIC TECHNIQUE Table 2 Chemical analyses Oxide S’iO2 A1203 TiO2 Fe203 MgO CaO Na20 K20 H20 H20" 65.21 11.6 n.d. 1.01 n.d. n.d. 3.85 3.44 n.d. n.d. 2 68.95 16.08 n.d. 1.65 2.12 0.86 3.08 5.34 n.d. 149 (mass %) of raw perlite of various origins 69.01 14.21’ 4 70.9 13.7 n.d0 0.22 1.45 1.9 0.05 1.9 3.6 3.4 3-4 n.d. 0.39 1.57’ 3.2 3.9 5.77 0.24 5 71.73 i3.64 n.d. 1.71 0.56 r.d. 3.55 4.15 3.47 n.d. 73.8 12.4 0.16 1.25 0.21 1.7 2.99 4.18 3.17 0.2 74:1 13.’ 0.0’5 0.5 0.1 0.6 3.5 4.6 3.5 0.1 H20 Loss on ignition at 800 C H20" Loss at 105 C n.d. * 3 5 7 not determined includes P205 and MnO Siberia [8] Korea [9] Yugoslavia [3] Sorroco (New Mexico) [11] 2 4 6 Sardinia [8] E1 Rosario (El Salvador) 10] Milos (Greece), the analysed perlite Mineralogically, perlite consists of a hydrated natural volcanic glass in which are embedded small amounts (up to 20%) of crystalline inclusions. Table 3 shows the mineral phases detected in perlites from various places all over the world. In particular, perlite studied in this work contains 89.4% by volume of the amorphous phase and 10.6% by volume of the crystalline phase consisting of plagioclase (5.5%), alkali feldspar (0.9%), biotite (3.0%), quartz (0.8%), magnetite (0.3%) and traces of chlorite. The physical characteristics of the mineral impurities are shown in Table 4. The commercial vnlue of perlite is determined, among other things, by the percentage of crystallinity. The most common contaminant in perlite is crystalline silica which may occur in a number of polymorphic forms, the most of which are quartz, cristobalite and tridymite. Experience has shown that cristobalite and D. HERSKOVITCH AND I.J. LIN 5o Table 3 Mineral phases detected in perlite throughout the world Place Mexico 11 E1 Salvador 10] Korea [9] Sardinia [8] Table 4 Mineral phases quartz, feldspar, biotite plagioclase, amphibole, hyperstene, ore plagioclase, biotite, opaque minerals, traces of hornblende, apatite, zircon and sanidine quartz, mica, feldspars Physical characteristics of minerals Mineral Formula Colour Plagioclase (NaSi,CaA1)AISi208 White to feldspar grey Luster Hard:aess Density Magnetic properties very weakly Vitreous, ,6.0-6.5 2.63paramagpearly 2.76 netic or diamagnetic Orthoclase KAISi308 Red. grey, white Quartz Colourless, Vitreous SiO2 Vitreous 6.0-6.5 netic all colours Biotite (’hiorite K_(Mg,Fe)2(OH.F).. :Blackl AISi300 brown (Mg,Fe)sAl_Si:O,0. Green. (OH)s brown ’FeO.Fe203 Iron black 2.5-2.6 Diamag- Pearly Pearly 2.0-’3.0 7 !2.652.66 2.73.1 2.0-2.5 Diamag-n etic Paramagnetic 2.7. Weakly paramagnetic Magnetite Metallic 5.5-6.5 4.95.3 Ferromagnetic tridymite are very rare in perlite. Because perlite is very high in total silica, any determination must be based on a technique sensitive only to the crystalline portion since the results cannot be checked by an analysis for total silica. Techniques which can discriminate between crystalline and amorphous silica include infrared spectroscopy, XRD, polarised light microscopy, differential solubility and heavy mineral separation. UPGRADING OF PERLITE BY A MAGNETIC TECHNIQUE EXPERIMENTAL Magnetic separation experiments were performed in two stages, viz. laboratory and pilot, with the aim of removing the iron-bearing minerals. The laboratory tests were carried out with the Frantz isodynamic separator on various granulometric fractions. The longitudinal slope of the cute was set to 160 and the cross slope 190 Table 5 shows the magnetic induction of the Frantz separator as a function of the electric current while the separation results are summarised in Table 6. Frantz isodynamic separator: data Table 5 1.4 1.5 Current I (Amperes) 0.41 0.81 Magnetic induction B (Gauss) 4000 8000 12000 14000 15000 1.2 Magnetic separation in Frantz isodynamic separator Table 6 Magnetic strength IA) 0.25 M NM M NM 1.25 1.00 0.50 M NM M NM Total 1.50 M NM M NM -20+35 0.6 99.4 0.6 99.4 1.3 98.7 0.9 99.1 0.8 99.2 5.7 94.3 -35+50 0.6 99.4 0.6 99.4 1.5 98.5 1.5 98.5 0.6 99.4 5.8 94.2 -50+70 0.9 99.1 0.7 99.3 1.5 98.5 1.2 98.8 0.7 99.3 6.3 93.7 -70+ 100 2.5 97.5 0.9 99.1 1.6 98.4 0.9 99.1 0.6 99.4 8.1 91.9 M magnetic fraction (wt. %) NM nonmagnetic fraction (wt. %) All the magnetic fractions removed in different passes and the non-magnetic fraction were examined under a polarising microscope. Distributions of minerals during the separation experiments for-35+50 mesh and -70+100 mesh classes are presented in Tables 7 and 8. D. HERSKOVITCH AND I.J. LIN 52 Table 7 Mineral distribution of various fractions in Frantz separator for -35+ 50 mesh Mineral Magnetic pass (A) Amorphous Feldspar phases Biotite Quartz 6.8 90.7 86.8 19.5 6.7 1.7 0.5 0.5 0.9 0.3 (vol. %) Magnetite Chlorite material 0.25 0.50 0.75 1.00 1.25 1.50 1.75 nonmagnetic 30.3 6.6 11.3 15.2 77.5 95.5 95.5 95.6 9.6 1.6 0.9 0.6 7.1 1.9 3.0 2.1 1.2 0.3 52.4 0.8 0.9 0.3 0.3 0.2 64.4 7.2 1.8 calculated from a total of 600 counted particles Table 8 Mineral distribution of various fractions in Frantz separator for -70+100 mesh Magnetic pass (A) Mineral Amorphous Feldspar Biotite phases Quartz (vo. %) Magnetite Chlorite material 0.25 0.5O 0.75 1.00 1.25 1.50 1.75 nonmagnetic 60.2 58.6 9.8 56.4 90.3 92.4 96.8 94.4 12.5 3.6 5.9 7.6 30.8 1.6 0.2 0.7 0.5 28.3 30.0 12.5 2.7 3,5 4,8 56.4 30.4 0.3 0.8 calculated from a total of 530 counted particles The pilot stage was conducted by Inprosys, Inc. (High-force magnetic separators) and Osna Equipment, Inc. (Permroll). Table 4 shows that the iron-bearing minerals are distinguished chiefly by their magnetic properties. Thus, their magnetic separation requires a multistage operation: first to remove ferromagnetic minerals and then to inter---separate the paramagnetic ones. Results of the pilot-plant stage are presented in Table 9 and 10, together with the test parameters. UPGRADING OF PERLITE BY A MAGNETIC TECHNIQUE Table 9 Magnetic separation tests by Inprosys, Inc. Test no./Product Feed Magnetic Non-magnetic Magnetic 2 Non-magnetic 2 2 Feed Magnetic Non-magnetic Magnetic 2 Non-magnetic 2 3 Feed Magnetic Non-magnetic Magnetic 2 Non-magnetic 2 Table 10 Roll speed eed % % [rpm] rate weight Fe203 150 3 100 7.17 1.25 7.00 2.98 89.86 100 6.20 1.10 0.69 1.25 8.90 3.58 90.22 100 6.85 1.20 0.73 1.25 7.30 3.21 89.94 1.20 0.71 130 160 5.8 130 150 4.6 125 Magnetic separation by Osna, Inc. Test no./Product Roll speed Weight [rpm] [%] Feed rate Fe203 [%] It/h/m] Feed Magnetic Magnetic 2 Magnetic 3 Non-magnetic3 2 Feed Magnetic Magnetic 2 Non-magnetic 2 3 Feed Magnetic Magnetic 2 Non-magnetic 4 Feed Magnetic Magnetic 2 Magnetic 3 Non-magnetic 3 (*) calculated 250 175 125 200 175 200 175 225 200 150 100 15.99 9.01 9.66 65.16 100 7.78 18.02 74.02 100 10.11 9.22 80.2 100 10.83 6.68 22.15 59.83 2.36 1.25 2.39’ 2.25 0.68 1.25 2.81’ 4.29 0.71 1.25 3.56’ 5.84 0.70 1.25 0.65 153 D. HERSKOVITCH AND I.J. LIN 154 DISCUSSION The purpose of the present paper is to present a new process of upgrading which enables the use of crude perlite as a raw material in the manufacture of coloured glass containers. A typical batch for such a production is composed of sand, limestone, soda ash and alumina as main ingredients, and several other materials such as feldspars, sodium nitrate, barytes, culler etc. Low-iron perlite constitutes a source of silica and also adds alumina and alkalis. Moreover, due to its amorphous nature, perlite has a more reactive melting behaviour than other materials used for this purpose. The main problem is the iron content which is usually greater than 1.0% (see Table 2). In particular, the analysed perlite contains 1.25% Fe203. Thus diminution of the iron content became the first task. The simplest and the cheapest engineering method for solving the problem is dry magnetic separation. Two reasons render this possible: contained in magnetite (ferromagnetic), and in biotite and chlorite (both paramagnetic, as stated in Table 4), and can be magnetically separated. Fe203 is Microscopic studies showed that if the material is crushed, liberation of minerals can be achieved to some extent. Dealing with perlite as a raw material for glass production, one has to take into account two other factors besides the iron content: Particle size distribution which influences both the melting process and the glass quality Do The water content. A customer must know that, out of one tonne of perlite he buys, 10 to 50 kg represent water content which will be lost upon melting the material. Besides, the water will cause expansion of perlite and attention has to be paid to the amount of material that must be introduced into a container in which the meltin takes place. UPGRADING OF PERLITE BY A MAGNETIC TECHNIQUE 155 In order to study the behaviour of minerals in a magnetic field of different strength, tests were made with a laboratory Frantz separator. Table 6 shows that better separation is achieved with smaller particles but the differences are of little practical significance. However, with the-70+100 mesh fraction, a bette liberation of particles was achieved, improving the results. Tables 7 and 8 show the mineral distribution of magnetic fractions (impurities) from two granulometric classes following separation at different values of the magnetic field strength. It was expected that in a weak magnetic field the ferromagnetic minerals would be removed and that when the magnetic field strength is increased, paramagnetic particles would be removed. Microscopic analysis showed, however, that magnetite had not been entirely liberated and still appeared as small inclusions in other minerals. This is why diamagnetic particles of the amorphous phase, feldspar and quartz (which contain inclusions of magnetite) had been removed with the magnetic fraction even at weak magnetic field intensities. With respect to biotite, most of it is removed at between 0.5 and 1.0 A, thus behaving like a paramagnetic mineral. It seems that a part of biotite has a high content of iron and can be separated at 0.5 A (see Table 7). This iron-rich biotite is found in +50 mesh granulometric fractions. However, even biotite was not entirely removed, remaining as inclusions in glassy grains. Another iron-bearing mineral in perlite is chlorite which is completely separated at field strengths lower than 1.75 A. Feldspars and quartz are removed in different magnetic passes due to the inclusions of magnetite or biotite which they contain. This is also valid for particles of glassy material. What determines the limit beyond which these particles cannot be separated is the size and the number of inclusions. It will be seen that by increasing the magnetic field strength above 0.75 A the quantity of the removed amorphous glassy material also increases. At 0.25 A and 0.5 A the main phases separated are magnetite and biotite, respectively. This is not valid for large---sized particles (+35 mesh) where good liberation was not achieved. Figures 3 to 7 show a succession of magnetic fractions removed on Frantz separator, and the final non-magnetic fraction of the-100+70 mesh fraction. UPGRADING OF PERLITE BY A MAGNETIC TECHNIQUE 159 It can be concluded that the size of particles affects the results not so much through achieving better liberation of minerals due to more finely crushed particles, as was thought at the beginning, but rather through proportion between the particle size and the inclusion content. During the pilot experiments the best separation of iron-bearing minerals was achieved by using the Permroll separator in a three-stage magnetic separation, at the feed rate of 5.84 tonnes per hour per meter width of the roll. The Fe203 grade of the non-magnetic fraction obtained with this device was 0.65%, at the material recovery of 59.83%. It will also be seen from Tables 9 and 10 that a better recovery (more than 80%) can be reached if the Fe203 content is reduced to as low as 0.69 0.70%. It will be shown later that this content is low enough to permit the use of raw perlite in the production of coloured glass containers. Besides, the laboratory experiments proved that the particle size has no considerable influence on the material recovery so that additional crushing will not improve substantially the results. The upgraded perlite obtained with Permroll was divided into three granulometric classes, in order to see if the size of particles has any influence on the mineralogical content. The results of the microscopic analyses are shown in Table 11. It can be seen that the more finely ground the product is, the more the impurity concentration decreases. Feldspar is abundant in larger sized classes while quartz is concentrated in smaller fractions. Biotite is found as inclfisions in amorphous glass particles. It seems that the presence of these inclusions is responsible for relatively high FeOa content in the non-magnetic product. Table 11 Microscopic analysis of the non-magnetic fraction from Permroll Particle size (mesh) -18+35 -35+50 -50 Amorphous phase 91.8 92.2 92.5 Mineral content (vol.%) Feldspar Quartz Biotite 0.5 1.2 2.4 0.9 0.7 0.4 6.8 5.9 4.7 D. HERSKOVITCtt AND I.J. LIN 16o The upgraded perlite was successfully tested at the Phoenicia plant, Yerucham, as a mineral additive for the production of coloured glass containers. Four specifications were established for perlite so that it can be used as a raw material in this industry: Chemical: Fe203 concentration less than the alkaline content of about 8% 0.7%; SiO2/AIO3 ratio and Physical: grain size of less than 18 mesh (1 mm) Amorphisation: reactivity of perlite Homogeneity and uniformity of the material. CONCLUSIONS io Its amorphous nature and chemical composition, together with its melting point make crude perlite a very suitable raw material for coloured glass industry. Its potential depends mainly on the iron content. FeO is found in magnetite, biotite and chlorite. Therefore, the simplest engineering solution for upgrading is the dry magnetic separation of these iron-bearing minerals. The basic unit operation has to be executed in several stages; first to scalp the ferromgnetics and then to remove the paramagnetic minerals. The magnetic separation experiments did not succeed in reducing the FeO content below 0.65%. The reason for this is the smallness of the inclusions of biotite and/or magnetite in the non-magnetic fraction. Perlite with iron content below 0.7 % and maximum grain size of 18 mesh (1 mm) can be used in the manufacture of coloured glass containers. UPGRADING OF PERLITE BY A MAGNETIC TECHNIQUE 161 REFERENCES [I] A. Petkova, L. Jonev and M. Marinov: Substitution of feldspar as a component of wall and floor tiles by other alkali---containing raw materials. Bol. Soc. Esp. Vidr. 29 (4), (1990), 249-252 [2] R. Niemann: Perlite-a new mineral filler for the promotion of sintering in the ceramic industry. ZI International, no. 7 (1991), reprint [3] S. Zafirovski, B. Jasmakovski, V. Zlatanovic and B. Pavlovski: Use of perlites in the ceramic industry. 2nd Int. Conf. on Natural Glasses, Prague (1987), p. 169-175 [4] P.L. Antonucci, M.L. Crisafulli, N. Giordano and N. Burriesci: Zeolitization of perlite. Mat. Letters 3(7), (1985), 302-307 S.S. Uluatam: Assessing perlite as a sand substitution in filtration. J. Amer. Water. Work Assoc. 83(6), June 1991, p. 70-71 [6] M. Schorr: The degree of corrosivity of phosphate ores. 11th Assoc. Advanc. Min. Eng. December 21-23, 1992, p. 242-243 [7] A. Buch: Ceramic glass production. 11th Conf. Israeli Assoc. Advanc. Min. Eng., December 21-23, 1992, p.35-42 [8] N. Burriesci, C. Arcoraci, P.L. Antonucci and G. Polizzotti: Physico---chemical characterisation of perlite of various origins. Mat. Letters 3(3), January 1985, p. 103-110 [9] J.H. Noh and J.R. Boles: Diagenetic alteration of perlite in the Guryongpo area, Republic of Korea. Clays and Clay Min. 37(1) (1989), 47-58 [10] W. Lorentz and P. M/iller: Perlite in E1 Salvador, Central America. New Mexico Bureau of Mines and Min. Resources, Circ. 182 (1982), 103-107 Conf. Israeli D. Herskovitch received her B.Sc. degree in 1990 from the Department of Geological Engineering and Geophysics, University of Iasi, Romania, and her M.Sc. degree from the Mineral Engineering Department, Technion. Mrs. Herskovitch is currently a D.Sc. postgraduate student at the Mineral Engineering Research Centre, Technion, Haifa, Israel. I.J. Lin: for biography see Magn. Electr. Sep. 3 (1992), 104 Keywords: Dry magnetic separation, perlite, raw materials, glass industry, ceramic industry